This application claims priority on Finnish Application No.20012514, filed Dec. 19, 2001, the disclosure of which is incorporated by reference herein.
Not applicable.
The invention relates to a dryer section of a paper or board machine.
As known from the prior art, the dryer section of paper or board machines has employed cylinder drying in which the dryer section is formed of drying groups. In cylinder drying, a paper or board web is dried against the heated surface of a drying cylinder, against which surface the web is generally pressed by means of a wire or equivalent. The drying cylinders are heated, for example, by steam.
The drying groups in the cylinder dryer sections of a paper or board machine employ twin-wire draw and/or single-wire draw. A drying group is formed by the drying cylinders, reversing rolls or cylinders, alignment and guide rolls associated with the same drying wire/belonging to the loop of the same drying wire in single-wire draw or associated with a pair of drying wires/belonging to the loop of a pair of drying wires in twin-wire draw.
In twin-wire draw, the groups of drying cylinders comprise two wires which press the web, one from above and the other one from below, against the heated cylinder surfaces. Between the rows of drying cylinders, which are usually horizontal rows, the web has in twin-wire draw free and unsupported draws which are susceptible to fluttering, which may cause web breaks, in particular in those stages of drying in which the web is still relatively moist and, therefore, has a low strength. For this reason, in the last few years, ever increasing use has been made of said single-wire draw in which each group of drying cylinders comprises only one drying wire on whose support the web runs through the entire group so that the drying wire presses the web against the heated cylinder surfaces of the drying cylinders and the web remains at the side of the outside curve of the reversing cylinders or rolls situated between the drying cylinders.
Thus, in single-wire draw, the drying cylinders are arranged outside the wire loop, and the reversing cylinders or rolls are arranged inside the loop. In what is known as normal single-wire draw groups, the drying cylinders are arranged in an upper row and the reversing cylinders or rolls are arranged in a lower row and, correspondingly, in what is known as inverted single-wire draw groups, the drying cylinders are arranged in a lower row and the reversing cylinders or rolls are arranged in an upper row.
The present invention relates to a dryer section of a paper or board machine, which dryer section applies the so-called normal single-wire draw and which, in addition to drying groups that apply single-wire draw, may also include drying groups that apply impingement drying or through-drying. In high-speed paper or board machines that apply single-wire draw, the first cylinder drying group has posed a special problem; it has been necessary to make it short in high-speed paper or board machines having a speed of, for example, over 1200 m/min when manufacturing fine paper, over 1400 m/min when manufacturing SC paper, over 1500 m/min, advantageously over 1600 m/min when manufacturing newsprint, over 1000 m/min when manufacturing fluting and liner, in order to be able to maintain sufficient tension in the web by means of differences in draw so that no runnability problems should arise. The difference in draw refers to a difference in speed both between a press section and a dryer section and between successive drying groups, in which connection, for providing tension, the following drying group uses a speed that is slightly different from the speed used in the preceding press or drying group. In that case, instead of one group, it has been necessary to place two or more cylinder drying groups at the beginning, which has increased manufacturing and operating costs.
At the beginning of the dryer section, the web does not yet contain a lot of solids, which means that its transfer from the press to the dryer section and its runnability in the first cylinder drying group have been problematic because, when the web is passed from a drying cylinder to a reversing cylinder or roll, the web may have separated from the wire and formed bags, the elimination of which has required a high draw value, which has led to the use of short cylinder drying groups, even groups formed by one cylinder, as the first group of the dryer section. The above-mentioned difference in draw from the press has been used specifically for eliminating the bags described above in order to make runnability good enough. The draw difference affects the quality properties of the web and, for the above-mentioned reasons, it has not been possible to optimize the draw difference taking into account quality properties.
With increasing running speeds of paper or board machines, it has been conventionally necessary to also arrange the long drying groups, for example, 8-9 cylinders, used for easy grades in prior art low-speed machines so that they form, for example, two drying groups in order to get runnability under control by thus providing separate drying groups that enable a difference in draw.
In prior art paper and board machines, the length of the first cylinder drying group has become shorter with increasing machine speeds. Although the solids content after the press has increased in recent years, the length of the first cylinder drying group has nevertheless continued to become shorter in prior art dryer sections that apply today""s technology.
In prior art cylinder drying groups that apply single-wire draw, the drying wire and the web come from a preceding drying element, for example, from a contact drying cylinder to a reversing suction cylinder or equivalent as a joint straight run, in which connection a closing wedge space is formed between the drying wire and the surface of the last-mentioned suction cylinder, the wedge space being also called a closing nip in the following. The moving drying wire and cylinder surfaces tend to induce overpressure into said wedge space. This in turn produces a pressure difference that affects the web supported by the drying wire and which tends to separate the web from the drying wire, causing runnability problems, wrinkles and even web breaks. On the other hand, in order to improve the efficiency of dryer sections, there is a need to use dryer sections that are more compact than previously and in which the contact drying cylinders and said suction cylinders are as close to one another as possible. All these matters together with increasing web speeds add to the overpressure problems in said closing nip.
A closing wedge space, i.e. a closing nip, i.e. a closing gap is formed when the wire runs towards the next suction cylinder/drying cylinder between the drying wire and the surface of said next cylinder. In a corresponding manner, an opening wedge space, i.e. an opening gap, i.e. an opening nip refers to a wedge-shaped space which is formed between the drying wire and the surface of the suction cylinder/drying cylinder and from which suction cylinder/drying cylinder the wire separates.
In the case of the opening gap, i.e. opening nip, a problem has been that the web starts to follow the drying cylinder and not the drying wire towards a reversing cylinder. The run of the web in this opening nip between the drying cylinder and the wire becomes the more difficult to control, the higher the paper or board machine speeds become, because with increasing speeds the web is more liable to follow the drying cylinder. The solids content of the web is also important for the separation of the web from the cylinder. The wetter the web, the more difficult is it to separate it from the cylinder. If the web does not follow the drying wire, the web will have slackness, which may cause wrinkles and other problems. The separation of the web from the cylinder and the supporting of it against the wire therefore requires increasingly more efficient methods when speeds are increased.
FI patent applications 19990370 and 19991908 disclose a runnability component which provides a high under-pressure on the rear side of the wire, i.e. on the side opposite to the web at the opening gap, and a lower under-pressure in a pocket space. FI application 19990370 discloses a blow device in a paper machine or another equivalent machine, such as a board or finishing machine, in which the web is conveyed while supported by a wire or equivalent over a cylinder, such as a drying cylinder or another roll, between the cylinder and the wire, and which blow device comprises a blow box or a blow box assembly which extends across the entire width of the web and which is connected to members that produce blowing air, and which blow device is disposed on the side of the wire facing away from the cylinder substantially at the opening nip between the wire and the cylinder to extend at least a small distance forwards from the nip in the running direction of the wire, and provided with at least two sealing members, such as a nozzle slot, an ejection nozzle, a mechanical seal or equivalent, which are transverse to the running direction of the web and disposed close to the wire, which sealing members are disposed in the blow device such that the first sealing member, which is a nozzle, is disposed substantially at the opening nip between the wire and the cylinder to blow air jets away from a gap between the wire and the blow device, and such that the second sealing member is disposed at a distance from this opening nip in the running direction of the wire to blow air jets away from the gap between the wire and the blow device or to limit the flow of air in this gap, whereby the sealing members maintain an under-pressure in the space between the blow device and the web. The blow device is additionally fitted with a throttling member at a small distance from the opening nip, which throttling member projects towards the wire and divides the under-pressure space formed between the first and the second sealing member into a first area of enhanced under-pressure confined to the location of the opening nip and into a second area of lower under-pressure. FI application 19991908 discloses under-pressure control arrangements associated with this.
When used at the beginning of a dryer section of a paper or board machine in demanding (speed, web grade) running conditions, problems have been encountered in prior art applications. One problem has been the differences in draw which are effective between groups. When the web being dried lies freely on the cylinder surface, its edges separate from support and thus the web requires increasingly more draw with increasing speed. In addition, one problem has been posed by the maintenance of web tension because tension disappears when a web of low solids content is stretched, with the result that bags causing runnability problems may be formed in the web. Moreover, the web may form bags when it has no tension and thus the web does not remain tight, which may also cause runnability problems. Stresses in the web are relaxed by the action of temperature, whereby web tension is lowered, which may further cause runnability problems.
In addition, the draw caused by draw differences may impair the other properties of the web, for example, its porosity, oil absorbency, Scott Bond (bonding strength) and roughness. It has not been possible to optimize these properties because runnability has formed the primary basis for determining the differences in draw.
With the conventional drying concept applying the prior art technology it has been problematic to control the properties of the web as desired during drying.
In paper or board machines known from the prior art, between the press and the dryer section, a draw difference of 2-3.5% has been used for fine paper when the running speed of the dryer section has been over 1350 m/min, when making LWC paper (Light Weight Coated) at a running speed of over 1600 m/min, the draw difference has been about 2.8% at its lowest and at a speed of 1800 m/min it has been about 3.1%, and when making newsprint at a running speed of 1600-1800 m/min, the draw differences used have typically been 2.8%, 3.3% and 3.5%.
Thus, in conventional drying concepts that apply known technology, the problematic areas have been the area of the opening nip, where problems have been caused by the fact that the web starts to follow the drying cylinder and does not accompany the drying wire, and the area of the closing nip, where an overpressure has caused the web to form a bag before the web moves onto the surface of the reversing roll, and the third problematic point has been the transfer over a group gap, in which it has not been possible to make the web run from a cylinder to a drying wire of the next drying cylinder group with a small difference in draw and, thus, with low tension, but the web has tended to follow the wire of the preceding group already before the location where the group changes or tended to follow the drying cylinder of the preceding group after the location where the group changes, and when a big draw difference is used, it may have caused problems with the quality of the web.
An object of the invention is to provide a high-speed dryer section of a paper or board machine having a better runnability than that of the prior art arrangements.
An object of the invention is to provide a high-speed dryer section of a paper or board machine in which no runnability problems are caused in the area of opening and closing nips nor in the transfer over a group gap.
An object of the invention is to provide a dryer section of a paper or board machine in connection with which the difference in draw between the press section and the beginning of the dryer section need not be determined based on runnability but the draw difference can be optimized based on quality properties.
The aim of the present invention is to develop further the technology corresponding to that described above such the problems caused by said opening nip area and closing nip area in the transfer of the web can be largely solved.
In particular, at high speeds at the beginning of a dryer section that applies single-wire draw, the area which is very critical in respect of runnability is the beginning of the dryer section, and a special object of the invention is thus to provide a cylinder dryer section of a paper or board machine in which the beginning of the dryer section does not suffer from the problems described above.
A special object of the invention is also to provide a high-speed dryer section of a paper or board machine in which the first cylinder drying group is long, which means that the web is not subjected to harmful additional draw differences, but the desired draw is arranged between the press and the dryer section by optimizing the quality.
An object of the invention is also to provide a dryer section of a paper or board machine in which there is no need for several drying groups at the beginning of the dryer section to produce draw differences in the web for runnability reasons.
In the paper or board machine in accordance with the invention, the first cylinder drying group is provided with runnability components by means of which it is possible to direct a high under-pressure of 1000-8000 Pa, preferably 2000-4000 Pa, at the opening gap on the rear side of the wire, i.e. on the side opposite to the web, and a lower under-pressure of 100-500 Pa, preferably 200-300 Pa, at another desired part of the pocket space or at all the rest of the pocket space. In connection with the invention, the rest of the pocket space refers either to the entire pocket space other than the area on the rear side of the wire at the opening gap or to a desired part of the rest of the pocket space, i.e., not necessarily the entire area of the rest of the pocket space. For example, a lower under-pressure can be applied only to that part of the wire run from the drying cylinder to the reversing cylinder which is not any more at the opening gap.
So high an under-pressure is directed at the gap opening on the rear side of the wire that the web is transferred easily without any problems, e.g. formation of bags, wrinkles, etc., from the cylinder together with the wire forwards. The magnitude of the under-pressure used naturally depends on running speed and on the paper or board grade being manufactured as well as on other running parameters of the paper or board machine.
In accordance with the invention, in the other drying groups after the first cylinder drying group it is also possible to use other types of conventional blow boxes. The runnability components, so-called high-efficiency blow boxes, used in the first cylinder drying group ensure a good runnability of the web immediately at the beginning of the dryer section when the web is still wet.
In the dryer section of a paper or board machine in accordance with the invention, the first cylinder drying group comprises at least five drying cylinders. In the dryer section of a paper or board machine in accordance with the invention, the number of the drying cylinders in the first drying cylinder group is maximized such that the upper limit for the number of drying cylinders is constituted by the situation in which the web starts to shrink to such an extent that a group gap is needed to achieve runnability past the group gap with a desired, preferably small draw difference, which may be even negative because of the machine direction shrinkage caused by the drying of the web, in which case the next drying cylinder group is thus driven more slowly than the preceding drying cylinder group. In that connection, between the press and the dryer section it is possible to use draw differences of, for example, below 2% at a running speed of over 1200 m/min and below 3% at a running speed of over 1600 m/min.
The number of drying cylinders used in the first cylinder drying group naturally also depends on the diameter of the drying cylinders used and, according to the basic idea, the long first cylinder drying group in accordance with the invention extends at its maximum to a point where the stress produced by shrinkage of the paper or board web starts to have an adverse effect on runnability, at which point the solids content of the web is generally 50-65%, preferably 55-60%, advantageously 55-65%.
The advantage achieved by means of the arrangement in accordance with the invention is, among other things, savings in that, when the first cylinder drying group is long (typically replacing at least two present groups), fewer tensioning/tightening means, guides and wire guide rolls and the like are needed, which means that the manufacturing and operating costs are lower.
In accordance with the invention, a dryer section of a paper of board machine is achieved which has good runnability and which substantially uses only a small draw difference, or needs no draw difference at all, between the end of the press section and the beginning of the dryer section, and thus the runnability of the machine is not a factor that limits the speed of the dryer section of the paper or board machine and its selection quantities of draw difference and other selection quantities, but, instead, it is possible to focus on the quality of the web and/or on desired running speed.
When the dryer section in accordance with the invention is applied, the differences in draw can thus be determined so that they are at optimum values from the standpoint of the quality properties of the web and the running speed, since the runnability problems have been solved by other means. In connection with the dryer section in accordance with the invention, the running parameters can be optimized, for example, based on the properties and beating degree of pulp. Savings in the consumption of energy are obtained by suitable selection of the beating degree.
The invention is particularly suitable, for example, when manufacturing a web from a short fibre chemical pulp or from recycled fibre containing pulps on high-speed paper or board machines.
In connection with the invention, a closed draw is preferably applied from the press section to the beginning of the dryer section.